1. Field of the Invention
The present invention relates generally to integrated optical ("planar") Bragg gratings, and particularly to integrated optical Bragg gratings whose peak reflection wavelength is substantially insensitive to temperature variations.
2. Technical Background
A Bragg grating consists of an optical waveguide defining a pattern or stack of regions having alternating higher and lower refractive indices, so that light within a narrow wavelength band is reflected by the grating, and wavelengths outside the band are transmitted through it. A Bragg grating is an excellent narrow-band optical filter which may have a variety of applications, such as a wavelength multiplexer, a fiber laser mirror, a dispersion control device, or a sensor. For this reason, Bragg gratings are becoming increasingly important in optical communications.
An important characteristic of a Bragg grating is the stability of its peak reflection wavelength with respect to variations in temperature. The peak reflection wavelength of a silica fiber Bragg grating generally increases with temperature by about 10 picometers/.degree. C., unless measures are taken to compensate for this shift. Stress may be used to provide such compensation. The peak reflection wavelength of a silica fiber Bragg grating tends to increase linearly with stress at approximately 0.1 picometers per psi of tensile stress. In order to stabilize the peak reflection wavelength of a fiber Bragg grating over a range of operating temperatures, various methods have been devised to vary the level of stress experienced by the grating as the temperature varies, so that the peak reflection wavelength is substantially temperature insensitive. As a rule of thumb, if the stress on a fiber Bragg grating decreases at a rate of about 120 psi/.degree. C., the grating's peak reflection wavelength will be substantially invariant with respect to temperature.
One way of achieving temperature compensation is to attach a fiber Bragg grating that is under tension to a substrate that has a negative thermal coefficient of expansion. As the temperature increases, the substrate contracts, thereby relieving some of the tension in the grating. When the fiber Bragg grating and the substrate are properly designed, the effect that this reduction in the tension in the grating would have on the peak reflection wavelength is offset by the effect that the increase in temperature would have on the peak reflection wavelength in the absence of any change in tension.
There are several drawbacks to the current technology, however. First, state of the art products are bulky and may need to be housed in a hermetic package to prevent degradation of the substrate and/or the fiber-to-substrate bonds. Secondly, the manufacturing process of current devices is generally complicated and expensive. Thus, there remains a need for a compact fiber Bragg grating that is relatively insensitive to variations in temperature.